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The crime is deceptively simple and maddeningly effective.

Thieves, often at night, use string to lower glue-covered rodent traps or bottles coated with an adhesive down the chute of a sidewalk mailbox. This bait attaches to the envelopes inside, and the fish in this case — mail containing gift cards, money orders or checks, which can be altered with chemicals and cashed — are reeled out slowly.

This low-tech crime, which became common in the Bronx, is known as mail fishing.

Now it has met its match.

The United States Postal Service is replacing or retrofitting mailboxes in much of the northeast to eliminate the pull-down handle in favor of a slender mail slot with a singular goal: foiling thieves.

All of the approximately 7,000 mailboxes in New York City and those in other parts of New York, New Jersey, Connecticut and Massachusetts are being revamped. No plans yet exist for a nationwide overhaul, officials said, but in areas where fishing becomes prevalent, the thief-proof boxes will likely follow.

“It’s safety, with anything. Twenty years ago, maybe you wouldn’t have locked your door, or locked your car. But as time changes, we all have to change with it,” said Donna Harris, a spokeswoman for the New York division of the United States Postal Inspection Service, the law enforcement arm of the Postal Service that works closely with local and State Police departments.

The mail slots are only large enough for letters, meaning sending even small packages will require a trip to the post office. The opening is also equipped with a mechanism that grabs at a letter once inserted, making it difficult to retract. (With an air of crime-fighting secrecy, postal inspectors declined to elaborate on precisely how the device works. “Those features on the box, what we like to call the Cadillac of mailboxes, those are things I can’t talk about,” Ms. Harris said.)

Most mail fishing thefts are done by lone thieves looking for quick cash. Though in some instances, the New York Police Department has partnered with the F.B.I. to take down organized crime rings that aim to not only steal money, but to commit identity theft, bank fraud and more.

“Those cases deal with thefts in the hundreds of thousands of dollars,” said Lt. Tara Deckert, of the Police Department’s grand larceny division.

In 2018, about 3,000 incidents of fishing were reported in New York City, up from 2,800 the year before, said Jason Savino, the grand larceny division’s commanding officer.

As of March 1, 352 mail fishing incidents had been reported in New York City this year, placing the city on track to record a 30 percent reduction in fishing from last year — a decline officials attribute to the newfangled boxes.

Still, with tax season in full swing, the police are warning snail mail users to remain vigilant. Postal inspectors and the police also suggest that people use gel pens rather than ballpoint on mailed checks to make them harder to erase, and to deposit mail just before a scheduled pickup time so that envelopes do not sit in boxes overnight.

“During tax season, there probably is a greater chance of catching fish, so to speak,” Officer Savino said.

Once boxes have been hit by thieves five or more times they are labeled “hot boxes” by the police and prioritized for surveillance or upgrades, Lieutenant Deckert said.

All of the mailboxes in the Bronx already have been replaced or retrofitted, and Manhattan’s overhaul will be complete by summer. Mailboxes in Brooklyn, Queens and Staten Island are slowly being upgraded.

Alexander Sylvester, a postal inspector with the Philadelphia division that also covers a large swath of New Jersey, said his office is trying hard to change “as many bins as possible,” but have prioritized areas most impacted by theft.

The introduction of the new boxes has led to some confusion.

One letter carrier in Midtown Manhattan said the volume of mail in the boxes he empties seems to have decreased by about half, and a few customers have asked him if the box was locked.

The Postal Service, which has been struggling financially for years in the face of declining mail volume, did not respond to questions about the average cost of each new or updated mailbox, or whether the overhaul had made an additional dent in mail volume.

Earlier this month, Mary Stella, who lives in Midtown Manhattan, peered and poked at the slot before inserting her letter. “The first time I used this box, I was confused,” Ms. Stella admitted, but added that she is a fan of the new design “because you can’t stick your hands inside and take things out.”

It is also not possible to insert a package containing an object that could cause harm, although preventing terrorism is not the primary goal.

“Obviously the boxes could have more than one purpose, but the boxes are being placed to thwart mail theft,” Ms. Harris said.

In addition to the new mail slots, some bins are being moved altogether for security reasons. Mailboxes that had been accessible for drive-by drop offs, for example, have been relocated next to post offices for better surveillance, officials said.

Even with the security upgrades, Margret Temple, of the Upper West Side, said she has not used a royal blue sidewalk mailbox for more than a year.

“I just go to the post office because it’s just safer,” said Ms. Temple, 65.